As radio frequency (RF), microwave, and millimeter-wave communication systems and devices continue to proliferate, there is an increasing need for more compact and efficient amplifiers in these frequency bands that can produce a desired output signal level.
FIG. 1 shows an amplifier 100 including first, second, third, and fourth gain elements 110-1 through 110-4, input power splitter 120, input resistor chain 140, output resistor chain 160, and output power combiner 180. Various conventional elements providing DC bias voltages and DC bias currents to first through fourth gain elements 110-1 through 110-4 are omitted from FIG. 1 for simplification of the drawing and the description to follow.
In a beneficial arrangement, first, second, third, and fourth gain elements 110-1 through 110-4 comprise substantially identical field effect transistors (FETs). However, other gain elements such as bipolar transistors could be employed instead.
Input power splitter 120 comprises: capacitor 121 connected between an input terminal 102 of amplifier 100 and a fixed voltage (e.g., ground); inductors 122, 123 connected between input terminal 102 of amplifier 100 and first and second splitter nodes 150 and 155, respectively; capacitors 124 and 125 connected between first and second splitter nodes 150 and 155, respectively, and the fixed voltage (e.g., ground); inductors 126 and 127 connected between corresponding input ports (e.g., gates) of first and second gain elements 110-1 and 110-2, and first splitter node 150; and inductors 128 and 129 connected between corresponding input ports (e.g., gates) of third and fourth gain elements 110-3 and 110-4, and second splitter node 155. Capacitors 121 and inductors 122 and 123 comprise an input stage of input power splitter 120 that couples input terminal 102 of amplifier 100 to first and second splitter nodes 150 and 155.
Input power splitter 120 is a symmetrical network. In particular, inductors 126, 127, 128 and 129 have substantially the same inductance values as each other, L1.
In this context, and throughout this disclosure, “substantially the same” means that the values are the same within the reasonable manufacturing tolerances of the device. By contrast, “substantially different” means substantially not the same, and therefore “more different” than the reasonable manufacturing tolerances of the device. In one embodiment, the tolerances of inductors 126, 127, 128 and 129 are such that all of the inductance values are within 5% of each other, and therefore are deemed to be “substantially the same,” while “substantially different” means that the values may be different from each other by 5% or more.
Input resistor chain 140 comprises: first resistor 142 connected between an input port (e.g., gate) of first gain element 110-1 and an input port (e.g., gate) of second gain element 110-2; second resistor 144 connected between an input port (e.g., gate) of second gain element 110-2 and an input port (e.g., gate) of third gain element 110-3; and third resistor 146 connected between an input port (e.g., gate) of third gain element 110-3 and an input port (e.g., gate) of fourth gain element 110-4. In general, resistors 142, 144 and 146 have substantially the same resistance values as each other, R1.
Output resistor chain 160 comprises: first resistor 162 connected between an output port (e.g., drain) of first gain element 110-1 and an output port (e.g., drain) of second gain element 110-2; second resistor 164 connected between an output port (e.g., drain) of second gain element 110-2 and an output port (e.g., drain) of third gain element 110-3; and third resistor 166 connected between an output port (e.g., drain) of third gain element 110-3 and an output port (e.g., drain) of fourth gain element 110-4. In general, resistors 162, 164 and 166 have substantially the same resistance values as each other, R2.
Output power combiner 180 comprises: capacitor 181 connected between an output terminal 104 of amplifier 100 and a fixed voltage (e.g., ground); inductors 182, 183 connected between output terminal 104 of amplifier 100 and first and second output combiner nodes 170 and 175, respectively; capacitors 184 and 185 connected between first and second output combiner nodes 170 and 175, respectively, and the fixed voltage (e.g., ground); inductors 186 and 187 connected between corresponding output ports (e.g., drains) of first and second gain elements 110-1 through 110-2 and first output combiner node 170; and inductors 188 and 189 connected between corresponding output ports (e.g., drains) of third and fourth gain elements 110-3 through 110-4 and second output combiner node 175. Capacitor 181 and inductors 182 and 183 comprise an output stage of output power combiner 180 that couples first and second combiner nodes 170 and 175 to amplifier output terminal 104.
Output power combiner 180 is a symmetrical network. In particular, inductors 186, 187, 188 and 189 have substantially the same inductance values as each other, L2.
In operation, amplifier 100: splits or divides an input signal (e.g., an RF, microwave, or millimeter-wave signal), received at input terminal 102, among the four gain elements 110-1 through 110-4 by means of input power splitter 120; amplifies the divided signal with each of the gain elements 110-1 through 110-4; and combines the output signals from the gain elements 110-1 through 110-4 by means of output power combiner 180 to produce an amplified output signal at output terminal 104. Input and output resistor chains 140 and 160 dampen oscillations that otherwise might occur with respect to gain elements 110-1 through 110-4 due to short-term voltage variations at the devices' output ports (drains).
Of significance, input power splitter 120 and output power combiner 180 serve not only to divide the input signal and combine the output signals of each gain elements 110-1 through 110-4, respectively, but also serve as impedance transformers to attempt to match impedances seen by the input ports (e.g., gates) and output ports (e.g., drains) of gain elements 110-1 through 110-4 to the corresponding impedances of the devices themselves, at the frequency(s) of the signal being amplified. More specifically, input power splitter 120 attempts to match the output impedance (e.g., 50 ohms) of an element supplying the input signal, to the input impedances at the input ports (e.g., gates) of gain elements 110-1 through 110-4. Similarly, output power combiner 180 attempts to match the output impedances at the output ports (e.g., drains) of gain elements 110-1 through 110-4 to an input impedance (e.g., 50 ohms) of an element (e.g., an antenna) receiving the amplified output signal from amplifier 100. Also of benefit, input power splitter 120 and output power combiner 180 are each configured in a low pass filter configuration so as to reduce any upper harmonic content of the signal that is being amplified (a bandpass filter configuration could be employed instead).
However, the aforementioned amplifier has some drawbacks. In particular, the impedance seen at the output ports (e.g., drains) of gain elements 110-1 through 110-4 is not the same for each device because the number of resistors in output resistor chain 160 that are connected to the output ports of gain elements 110-2 and 110-3 is different from the number of resistors connected to the output ports of gain elements 110-1 and 110-4, creating an unbalanced load condition for gain elements 110-1 through 110-4. As a result, amplifier 100 exhibits an early compression problem where the power level of the amplified output signal at output terminal 104 is less than expected. In particular, the power of the amplified output signal at output terminal 104 does not reach the expected output power level which should be approximately four times the output power level of each gain element, whose output power levels should all be approximately the same as each other. This problem may be exacerbated to a minor extent by the unbalanced loads at the input ports (e.g., gates) of gain elements 110-1 and 110-4 due to input resistor chain 140.
An additional resistor could be added to output resistor chain 160 to connect the output ports of gain elements 110-1 to the output port of 110-4, but this is undesirable because of space and routing limitations. Also, a similar degradation occurs, but to a lesser effect, due to the unbalanced conditions at the input ports of gain elements 110-1 and 110-4 because of different numbers of resistors of input resistor chain 140 being connected to the input ports of gain elements 110-2 and 110-3, versus elements 110-1 and 110-4.
What is needed, therefore, is an amplifier with a greater output power level, given the same individual gain elements. What is also needed is an amplifier including an output power combiner that provides an optimum load condition to all of the individual gain elements.